The present invention relates to a lens unit, and more particularly, to a lens unit suitable for use in a compact image capturing device. Further, the present invention relates to an image capturing device.
Most of image capturing devices that are presently being used are digital cameras, which use charged coupled device (CCD) image sensors or complementary metal oxide semiconductor (CMOS) image sensors as image capturing elements. Thus, such an image capturing device must use a lens unit of which capacities differ from that of a so-called silver salt camera. For example, the sensitivity to diagonal incident light is lower in a CCD image sensor or CMOS image sensor than in a silver salt film. Thus, a CCD image sensor or CMOS image sensor must have a small chief ray angle (CRA), which is the incident angle of light rays entering an image capturing device. Due to the improvements made on the integration of image capturing elements, the image quality must be improved even for lens units used in compact camera or mobile phones. At the same time, such a lens unit must be reduced in size. Furthermore, a lens unit that is compact and used in a compact camera or mobile phone must have a structure that lowers manufacturing costs so that relatively inexpensive products can be supplied.
To improve the image quality, aberrations must be lowered. Thus, instead of a lens unit having a three-lens structure, which may easily be designed to be compact, it is desirable that a lens unit having a four-lens structure be used, which more easily suppresses aberrations. Japanese Laid-Open Patent Publication Nos. 2002-228922, 2003-255222, 2005-91666, and 2006-309043 describe compact lens units having four-lens structures.
Japanese Laid-Open Patent Publication Nos. 2002-228922 and 2003-255222 each describe a so-called front stop lens system in which the stop is located further toward an image capturing side (hereinafter referred to as the “object side”) from all the lenses of the lens unit. In such a structure, it is generally known that when misalignment occurs in a lens located toward an image capturing element side (hereinafter referred to as the “image side”) from the stop, the misalignment greatly affects the image quality. As a result, there is a tendency for product defects to occur due to such misalignment. This lowers the manufacturing yield of the lens unit and increases manufacturing costs.
Japanese Laid-Open Patent Publication Nos. 2005-91666 describes a so-called middle stop lens system in which the stop is located between the furthermost object side lens and the second lens from the object side. It is generally known that in such a structure, misalignments less affect the image quality than the front stop lens system. However, product defects caused by misalignments cannot be sufficiently prevented.
Japanese Laid-Open Patent Publication No. 2006-309043 recites in claim 1 “an image capturing lens comprising a first lens having a negative refractive index and including a convex surface formed on an object side, a second lens having positive refraction power and including a convex surface formed on an object side and joined with the first lens, an aperture stop having a predetermined aperture, a third lens having positive power and including a concave surface formed on the object side, and a fourth lens having negative refractive power.” Accordingly, the lens system disclosed in this publication has a middle stop structure in which a stop is arranged between lenses that are second and third from the object side. It is thus considered that this structure sufficiently prevents product defects caused by misalignments. The publication also explains that “employment of a three-group, four-lens structure obtains a lens capturing lens that allows for reduction in size, thickness, and weight, achieves satisfactory optical characteristics for infinite to close-range image capturing, and is suitable for a high-density solid state image capturing element of 4,000,000 pixels or greater” (refer to paragraph [0014] of the publication.
However, in the lens unit described in Japanese Laid-Open Patent Publication No. 2009-309043, the two lenses located toward the object side from the stop includes the first lens having a negative refractive inject and the second lens having positive refractive power. Thus, the condensing of light through refraction tends to become insufficient. To cope with this problem, if the positive refractive power of the second lens is increased, the thickness of the second lens increases. This, in turn, increases the length of the entire lens unit.